1. Field of the Invention
This invention relates to a driving circuit for a liquid crystal display (LCD) apparatus, and more particularly to a driving circuit for an LCD apparatus having an active matrix type LCD panel.
2. Description of the Prior Art
Attention is directed to the related co-pending prior U.S. patent application Ser. No. 07/631,699 filed Dec. 19, 1990 to Nakagawa et al. entitled "A Driving Circuit For A Liquid Crystal Display Apparatus."
Generally, a conventional driving circuit for an LCD apparatus produces AC video signals from input DC video signals, and supplies the AC video signals to source lines of an LCD panel of the LCD apparatus. More specifically, as shown in FIG. 5, input video signal is supplied to a polarity-inverting circuit 41 through a buffer 42. The polarity-inverting circuit 41 alternatingly inverts the polarity of input video signals for each field. Namely, the polarity of video signals output from the polarity-inverting circuit 41 and supplied to an LCD panel is positive for odd fields, and negative for even fields, or vice versa.
FIGS. 6 and 7 show the input-output characteristics of the buffer 42 and polarity-inverting circuit 41, respectively. As shown in FIG. 7, the input-output characteristics of the polarity-inverting circuit 41 is offset toward the positive side by a constant DC offset voltage V.sub.offset. This DC offset voltage is produced so that the level of the DC component of video signals supplied to the LCD panel can be reduced as low as possible.
The reason why the DC component is to be compensated or canceled by the constant DC offset voltage will be described. FIG. 8 shows an equivalent circuit diagram of a picture element (pixel) of an active matrix type LCD panel in which thin film transistors (TFTs) are used as switching elements. A TFT 71 is disposed at each of crossings of a source line 72 and a gate line 73. The source and gate of the TFT 71 are connected to the source line 72 and gate line 73, respectively. The drain of the TFT 71 is connected to a pixel electrode 74 which opposes a counter electrode 75. Between the pixel electrode 74 and the counter electrode 75, a supplemental capacitance C.sub.S is formed in addition to a capacitance C.sub.LC caused by the liquid crystal layer disposed between the pixel electrode 74 and the counter electrode 75. Between the gate line 73 and the pixel electrode 74, furthermore, there is a capacitance C.sub.gd. When the pixel is to be driven, a scanning pulse .DELTA.V.sub.G is applied to the gate line 73. To the pixel electrode 74, therefore, applied is the following DC voltage .DELTA.V.sub.DC : ##EQU1## This means that the voltage of the pixel electrode 74 is biased by .DELTA.V.sub.DC with the application of the scanning pulse .DELTA.V.sub.G to the gate line 73. Therefore, a constant DC offset voltage is added in signals which are applied to the source line 72 or the counter electrode 75, thereby compensating the DC voltage .DELTA.V.sub.DC.
Owing to the anisotropy in the dielectric constant of the liquid crystal, however, the capacitance C.sub.LC of the liquid crystal layer changes as shown in FIG. 9 with the change of the voltage V.sub.LC applied to the liquid crystal layer, resulting in that the DC voltage .DELTA.V.sub.DC varies as shown in FIG. 10. Therefore, the application of a constant DC offset voltage cannot completely compensate the DC voltage .DELTA.V.sub.DC for each pixel. This incomplete compensation of the DC voltage .DELTA.V.sub.DC causes the problems such as the residual image phenomenon which impairs the image quality, the increased deterioration of the LCD panel which reduces the reliability, etc.